John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to see why
this should be true.

Because for the same Q value, the pass band would be 4 times wider

Where is it written that the same loaded Q must be used for both filters?
If you can change the center frequency, why can't you change the loaded Q?

The lower the Q, the more IFTs required for a given amount of pass band and
attenuationout of band.



Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.

Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.

So you have tried all that and rejected the "pointy nosed", "flat topped",
and "rabbit eared" response curves. I am left to wonder what sort of
response curve you were looking for? Why not settle for a nice "flat
topped" response curve and be done with it?

I didn't say I had rejected the flat topped critical coupled IF response.
That's what I use when my radio has the IFT coil distance adjust control set to
minimum BW, but when the coils are slid towards each other after tuning, the BW
becomes wider
without going rabbit eared, because the sum of the responses of the two IFTs is
still
flat topped. I found the mecanical slide method to be more predictable than any
other, and the IFTs don't
drift off Fo, and the rabbit ear shape in IFT1 is symetrical each side of Fo.

Commercially made communication receivers I have seen use the same method to alter
the coupling and hence BW of their three IFTs. There are no long wires going to a
complex switch.

Patrick Turner.



Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/

Jon Noring wrote:

Patrick Turner wrote:
Volker Tonn wrote:
Jon Noring schrieb:

In the last couple of years I've posted various inquiries to this and
related newsgroups regarding high-performance, tube-based AM (MW/BCB)
tuners, both "classic" and modern.

Have a look into the "Collins" S-series. These are state-of-the-art
tube sets 'til now. At least it's not the tubes alone but the fabulous
mechanical IF-filters giving outstanding results for a tube set.
Manuals with layout diagrams should be available on the web....

Since Mr Noring says he has regularly trawled the Net for everyone
else's expertise on AM reception, but got nowhere, because he's
still doin it, why doesn't he gird his loins and put his shoulder to
the task of learning all about AM and radio engineering as spelled
out so clearly in all the old text books, and then damn well build
his own perfect AM radio???

Thanks for sharing your frank coments. They are acknowledged.

The important thing is that the replies to my "trawling" have been
very informative, including yours Patrick, and are not only
benefitting me, but are benefitting many others who are following this
thread in real time.

Whether my trawling is successful or not for my purposes is immaterial
-- if I fail, I fail -- I don't fear failure as some do -- the
discussion is further adding to the information pool for the community
of those interested in some aspect of tube-based AM tuners, and in
that regards I think it has been successful. (With Google archiving
the newsgroups, this information is now being preserved, and is
searchable.)

I don't mind sharing whay I know, but you will be the one left to
decide what works or doesn't work, so you shoukld get away from the PC,
and ito the workshop to try out ideas mentioned in all the responses to your
query.
I for one haven't time for the R&D, but if I was more passionate about
good AM than I already am, it'd be to the workshop I would go,
armed with ideas, and solder on towards lower thd and more BW.



There are obviously two sides to the engineering of radio receivers:
1) the basic theory and the basic categories of design approaches
(which I am studying -- it helps in that back in 1974 I had the
equivalent of one years' worth of basic electrical engineering courses
at the University of Minnesota, which is now all coming back to me),
and 2) the real-world engineering of receivers/tuners, using real
rather than theoretical components, and the attendant compromises and
work-arounds which inevitably result.

I do agree with Patrick's implying that there is no such thing as a
"perfect radio". I am not seeking the "perfect radio", but a
modern-design tuner "kit" sufficiently meeting the various
requirements I have previously set forth. I believe once a good
design results, that PCB boards can be made, coils can be built by
someone or some company experienced in doing that (I mention coils
since that is the one component difficult to buy right off the shelf
-- thank god no one has to build their own tubes!), and the schematic
with detailed instructions and guidelines sold through diytube (as an
example.)

There already have been several excellent SS kit designs for decent AM
released by Oz makers
in past years, but its 35 years since any tube based kits were available.

Nobody seems to think it'd be commercially viable to present yet another
AM kit, because 90% of folks listen to FM.

So you are on your own wanting to make a kit design that could be sold,
and I wish you well with the prototyping of coils and circuits.




The target market for the "kit" are those who build their own tube-
based components for their audio system, and want every component to
be a high-performer, approaching audiophile-grade in performance (yes,
AM broadcasts are not "audiophile", but audiophiles want a tuner that
brings out the best in what is there in the signal.) They don't want
to spend their limited time building junk, they don't want to build a
Radio Shack beginners' crystal set. They want very good performance
(which is admittedly a "fuzzy" word), commensurate with their other
components. They just want the tuner kit not to be overly complicated
in design, to work if they follow the instructions and guidelines, and
to meet their (collective) expectations.

And these kit builders are not novices, either, at wielding a
soldering gun, and in chassis and cabinet design -- they are
mechanically- and electronically-inclined, and are now building
audiophile-grade amps and preamps from the many kits now out there. I
also believe that some of the vintage radio collectors, who are
experienced at restoring radios, will also take an interest in the AM
tube tuner kit. (For those who don't know, I'm now restoring a Philco
37-670 console, so I'm not exactly out-of-touch with the radio
collecting world.)

Based on my experience with building audiophile-grade tube amps and
plugging into that community, I think I've laid out pretty well what
they want and expect. Most are not going to become radio design
enthusiasts, they will not live and breathe tuners, building hundreds
of circuits on cake pans in their basement (and I am not disparaging
those who do!) They simply are going to listen to the tuner they
laboriously built from the kit, happy with its performance, and happy
for what they have learned about how radios work "under the hood", in
a general sense. Some will no doubt get the radio bug, and join the
people here, rescuing old radios from the landfill, and restoring
them.

Maybe my focus on TRF-based designs and "channel-based design" have
been diversions. But, from what I've read about real-world TRF designs
(John Byrns messages have been great here), a TRF-based design has
some nice attributes from the audiophile kit perspective, and there
are clever real-world solutions around the selectivity and gain
limitations of the "what's taught in textbooks" regarding basic TRF
design, as John Byrns and others have noted many many times, but which
seems to fall on deaf ears of those who believe that the best
high-performance receiver (however "high-performance" is defined)
*must* be super-het in basic design.

But obviously, the vast majority of commercial designs of
high-performance tube-based radio receivers from the mid 1930's to the
1950's are super-het designs, and many of them are great performers,
so I'll post a parallel message with another call for candidate radios
to inspire the AM tuner kit. After all, if one is to put together an
AM tube tuner kit, it makes a lot of sense to base it on a proven
design from the past -- why reinvent the wheel?

For example, diytube (at http://www.diytube.com/ ) has taken the
venerable Dynaco ST-35 amp design, modernized it some (and to further
improve its audiophile performance), and is now selling the PCB
board with schematics and instructions to diy audiophiles. it is an
excellent performer (I know firsthand -- it is a *very nice* sounding
amp.) diytube is now working on a high-power monoblock tube amp kit
based on the Eico EL34 amp of old -- can't wait until it is released.

Just some thoughts...

Jon Noring

We await with ardent expectations of the fruit of your your efforts in your
workshop
with a saleable prototype tube based AM radio tuner kit.


Patrick Turner.

Since the superheterodyne patents either started being licensed at
reasonable rates or ran out, few receivers of any other type have been
built. With very good reason.

PCB construction makes more sense for IF strips than for pure
baseband hardware when tubes are employed, but I don't know that
there's a big advantage to doing it with tubes unless you just like to
work with tubes. Doing it with FETs might make more sense. Still, very
good RF boxes were built before the PCB days.

I think you should get some coil components, which are still
available, and either a noise generator and a spectrum analyzer (or
one with a track gen...) or better yet a network analyzer , which will
show both transmission and reflected paths, and just decide what kind
of "haystack" you want, and cobble to suit. RF software exists so that
you can play with precise parms of I and C, but you will be happier
with the cut and try given stray inductance and capacitance and other
variables at 10.7 MHz (or whatever IF you wind up with.) People once
did it without these tools but then it took prodigal amounts of time
and they had techs and test operators who worked cheap.

When you get done, you will have the best fidelity of current AM
broadcast signals available. However, considering as how they've been
Orbanned into submission for "dial punch", and considering commercial
broadcast at least in the US sucks **** through a straw right now
content-wise, it may be a wholly Pyrrhic victory.

Brian wrote:

Take a look at the screen shot again. Don't the extremely steep
spectral walls--the sudden drop of tens of dB at 10 kHz--suggest
something to you?

Yeah - typical processing -- looks like what I would expect with very
good (NRSC) processing.

Let me try and explain it this way: Notice that the signal at 7.5khz is
down roughly 6db from the lower frequency (2-3khz) material. You have to
remember, then -- that makes that 7.5khz material only roughly 1/2 the
volume of the lower frequency (2-3khz) material. The difference here may
not look like much (as it's in db) but it's only HALF as loud. And as
you've noted - it drops off (as best this equipment can do) like a brick
wall past 9.5khz.

The inner spectral detail is interesting, especially
since the lopsided spectrum did not occur later in the day on
different program material

As noted - they probably were really punching talk material. Remember -
you can go way over 100% positive modulation - all commercial
transmitters are designed to do so. You just can't go past 100% negative
(how do you get less than 0 carrier? you don't - you get clipping /
splatter). In fact - most processors won't try to push past -95% -
leaving a "just in case" margin for OOPS!). Sometimes such un-symetrical
modulation causes some strange effects in the sidebands.

The better model optimods can be pre-programmed with many presets - and
chosen as desired. It's all "demographics" now - knowing and appealing
to the (perceived) audience - so changing "the sound" during a broadcast
day is now as routine as sponsors tailoring their ads to suit drive time
commuters, then soccer moms, etc.

(see http://n2.net/k6sti/later.jpg), but
it's the spectral boundaries that tell the story.

Well - let's see -- (relative to 2.5-3khz) they're -6db at 7.5Khz;
rolling on off to about -10db by 10khz - then about another 20db by
12khz. What's your point- looks like NRSC through a pretty good
processor - roll-off pretty much what we've been talking about...

I do note the "spike" in the positive sideband at about 8khz or so - I
feel that is probably a noise spike of some sort - especially since
it's so prominent out of the surrounding "curve"; and also it has NO
correspondence in the negative sideband at all - though it's hard to
tell from a snapshot.

That spectrum was recorded during classical music with little
high-frequency content, hence the sloping spectrum.

Yeah- but even so NRSC pre-emphasis is up to 10db by 10khz - which is
still getting rolled as it approaches the "brick wall".


What's interesting is where it suddenly vanishes at 10 kHz.

Look closer - it's not absolutely a "brick wall". There is some gentler
roll-off before 9khz - then it gets serious. Again - that's what current
state of the art looks like -- consistent with the NRSC spec - and what
I've said about filters / processing.

Again - you don't seem to have a good grasp of what you're looking at in
relation to what you hear. There is no way LA stations (or any other
AM band broadcasts in the U.S.) are going to be "flat" out to 10khz (not
these
days - some (clear channels) used to). It's been noted that the better
optimods (like the 9200) can go way out filter settings (9.5khz) -
Optimods so set would be very useful for ShortWave - and places where
the FCC (or equiv. authority) aren't so strict. I wouldn't recommend
them being so set in LA.

Back to your "music" example - What are they (the station's sidebands)
AT 8-9khz compared to the average at 2-3Khz? That roll-off is the effect
of the filters/processors doing their job in forming (as best they can)
the "brick wall" you do indeed see above 9.5khz.

The picture of "noise" tells us nothing - other than 10khz is down 30db
from carrier. Were it relative to something - that would tell us much.
As it is - it just shows an unknown spectrum of ???? Put a reference in
there (or a known weighted/gated noise source such as specified by NRSC
- like a BruelKjar or equiv.) THEN you can make some solid deductions.

If the material in the 2-3Khz ranges runs at / around -15-20db then
10khz is DOWN roughly 10-15db below that - which is ONLY
(approximately) 1/4 as loud. But since we don't know what the source
material is doing - it's meaningless.

I know dealing in dbs is confusing - but if you're going to deal with
Audio/RF/Spectrum - you HAVE to learn dbs and be familiar with how they
relate to what you hear.


best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

Randy and/or Sherry wrote:
I know dealing in dbs is confusing - but if you're going to deal with
Audio/RF/Spectrum - you HAVE to learn dbs and be familiar with how they
relate to what you hear.

Jeffie hands you another bucket of pearls to cast before the swine.

Jeff

--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"

The picture of "noise" tells us nothing - other than 10khz is down 30db
from carrier. Were it relative to something - that would tell us much.
As it is - it just shows an unknown spectrum of ???? Put a reference in
there (or a known weighted/gated noise source such as specified by NRSC
- like a BruelKjar or equiv.) THEN you can make some solid deductions.


That spectrum was taken during a quiet piano passage with background
noise. The piano, played softly, had little treble, so the spectrum
above about 3 kHz is the product of the program noise spectrum, and
the spectral response of the station, which includes playback
electronics, processor, transmitter, and antenna. The dominant
spectral feature of the station's frequency response is the processor
preemphasis. If the noise spectrum is flat, what you see in the screen
shot is the preemphasis curve. Its absolute level reflects the level
of the background noise, which isn't relevant. But the shape is. The
curve shown is typical of the spectral response you'd expect to see
for a preemphasized AM transmitter. The key point is that it stops
suddenly at 10 kHz, not somewhere below. All of the spectra I've shown
do the same. (The spectra of the two Mexican signals stop at 8 kHz.)

Here's a final screen shot: http://n2.net/k6sti/am1210.jpg . This is
nearby station at 1210 kHz that was broadcasting a live announcer from
a local studio at the time I recorded the spectrum. The carrier is at
the left edge of the screen, the center of the screen is 1220 kHz, and
the horizontal scale is 2 kHz/div. This image shows the upper sideband
in some detail.

If you were designing a high-performance AM receiver, what IF passband
would you use to fully recover the modulation from this signal?

Brian

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to
see why
this should be true.

Because for the same Q value, the pass band would be 4 times wider

Where is it written that the same loaded Q must be used for both filters?
If you can change the center frequency, why can't you change the loaded Q?

The lower the Q, the more IFTs required for a given amount of pass band and
attenuationout of band.

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are. There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

One One
455 kHz IFT 2.0 MHz IFT
Q = 15.167 Q = 66.667

Fc-60 kHz -24.30 dB -24.18 dB
Fc-50 kHz -21.22 dB -21.09 dB
Fc-40 kHz -17.56 dB -17.42 dB
Fc-30 kHz -13.22 dB -13.07 dB
Fc-20 kHz -8.72 dB -8.60 dB
Fc-15 kHz -7.09 dB -7.02 dB
Fc-10 kHz -6.27 dB -6.24 dB
Fc-05 kHz -6.04 dB -6.04 dB
Fc kHz -6.02 dB -6.02 dB
Fc+05 kHz -6.03 dB -6.03 dB
Fc+10 kHz -6.19 dB -6.22 dB
Fc+15 kHz -6.86 dB -6.96 dB
Fc+20 kHz -8.34 dB -8.50 dB
Fc+30 kHz -12.75 dB -12.95 dB
Fc+40 kHz -17.15 dB -17.32 dB
Fc+50 kHz -20.88 dB -21.01 dB
Fc+60 kHz -24.01 dB -24.12 dB

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.

Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat
topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.

So you have tried all that and rejected the "pointy nosed", "flat topped",
and "rabbit eared" response curves. I am left to wonder what sort of
response curve you were looking for? Why not settle for a nice "flat
topped" response curve and be done with it?

I didn't say I had rejected the flat topped critical coupled IF response.

Then what did you say? You said you had "tried all that" but now it
appears that you were telling a little fib and hadn't actually tried a 455
kHz IF designed to produce the desired response.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

"Sam Byrams" wrote in message
om...


When you get done, you will have the best fidelity of current AM
broadcast signals available. However, considering as how they've been
Orbanned into submission for "dial punch", and considering commercial
broadcast at least in the US sucks **** through a straw right now
content-wise, it may be a wholly Pyrrhic victory.

I have a wideband AM radio, and the some stations sound better than others.
One of the Polish language stations sounds pretty good in wideband. I'm
sure my Grandmother would have loved hearing good fidelity polkas, but I'd
prefer jazz. A couple of weeks ago they played a Classical recording before
sign-off which sounded great. The sound was well balanced and had lots of
dynamic range.

The gospel station sounds over treble boosted on my radio, but other wise
it's not bad. The oldies station is a bit of a disappointment. They have
great material, but it sounds a bit flat.

There are a couple of stations which don't sound very good.

I made a mistake in my previous post. I got off on a "testbed" tangent. It
would have been better to say "test drive". Wideband AM isn't necessaraly
bad, it can be pretty good. But I wouldn't get into a big project unless I
gave the local stations a good listen in order to judge if it's really
worthwhile.

Frank Dresser

Brian wrote:

The picture of "noise" tells us nothing - other than 10khz is down 30db
from carrier. Were it relative to something - that would tell us much.
As it is - it just shows an unknown spectrum of ???? Put a reference in
there (or a known weighted/gated noise source such as specified by NRSC
- like a BruelKjar or equiv.) THEN you can make some solid deductions.

That spectrum was taken during a quiet piano passage with background
noise. The piano, played softly, had little treble, so the spectrum
above about 3 kHz is the product of the program noise spectrum, and
the spectral response of the station, which includes playback
electronics, processor, transmitter, and antenna. The dominant
spectral feature of the station's frequency response is the processor
preemphasis. If the noise spectrum is flat, what you see in the screen
shot is the preemphasis curve. Its absolute level reflects the level
of the background noise, which isn't relevant. But the shape is. The
curve shown is typical of the spectral response you'd expect to see
for a preemphasized AM transmitter. The key point is that it stops
suddenly at 10 kHz, not somewhere below. All of the spectra I've shown
do the same. (The spectra of the two Mexican signals stop at 8 kHz.)

Here's a final screen shot: http://n2.net/k6sti/am1210.jpg . This is
nearby station at 1210 kHz that was broadcasting a live announcer from
a local studio at the time I recorded the spectrum. The carrier is at
the left edge of the screen, the center of the screen is 1220 kHz, and
the horizontal scale is 2 kHz/div. This image shows the upper sideband
in some detail.

Would not the use of pink noise through a low pass filter
and used as the carrier signal modulation be a better way to see the
frequency contour on an analyser, why noise + piano?



If you were designing a high-performance AM receiver, what IF passband
would you use to fully recover the modulation from this signal?

Brian

There are 5 divisions where there seems to be a signal, so to get
the 10 kHz of AF BW involved so that the 10 kHz response was 1 dB down at
10kHz,
about 30 kHz of IF BW would be required, ie, 15 kHz each side of the IF centre
F

This would be somewhat difficult using normal high Q 455 kHz IFTs.

I think one might have a much better chance if one used 2 MHz IFTs,
perhaps 3 of them, and settled for -3dB at 10kHz each side of 2MHz centre F.
Then an emphasis RC filter could boost the 10 kHz back up a bit.


In 1982 in the Australian magazine Electronics Australia, there was an
elaborate AM tuner kit
offered for sale for aud $250 back then which is about equal to usd $1,400
now.
It had 10 different coils types including 3 well damped 455 kHz IFTs, RF
coils, and 9 Khz whistle filter,
5 j-fets, 6 opamps, one ceramic filter, and 3 signal transistors, a 3 gang
variable tuning cap,
and lots of diodes, and R&C bits, and that doesn't include the +15v PS.

The set had non tuned RF input coil feeding 1st RF LC, then fet RF amp,
2nd RF LC, then two untuned balanced transformers and a two fet PP balanced F
converter
feeding IFT1, a fet IF amp, IF2, a 2nd IF fet amp, then IFT3.
The oscillator has a three winding coil, and 3 bjts.
The AF detector is a CA3016 with shunt FB to linearise the detection.
AVC is via TL071, 741, and UAA180m is used to drive
sigal leds and tuning meter.
Output audio is filtered by two TL072 and
with a passive bridged T filter.

I doubt that any of the coil components would be findable today.

The final AF response was - 3 dB at 10 kHz on the "wide" bw setting.

A tube kit to do the same thing today would cost at least the same shirtload
of money,
probably more.

Imagine trying to build any tubed radio today in small batch numbers, and in
doing so
include for an extra 3 tubes to achieve the low thd and wide BW
of the 1982 EA circuit.
It would make the cost greater than a tube power amp.

My paper files have around 20 different circuits for AM tuners including
a fairly simple synchrodyne ( or direct conversion ) two tube sets which
use a 6EJ7 for an RF amp, and followed by a 6BE6 synchronous detector.

I tried building one, but audio output was low, stability was difficult,
and and a superhet proved far better.

Then there were several if not many synchrodyne and some homodyne designs in
Wireless World
over the years, but these were all chip based, except the early
D.G.Tucker circuit of 1947.

Mr Noring wants some miraculously simple cheap design solution to drop out of
the sky.
He should pray to the God of Triodes, perhaps He will send a schematic
directly.

But then perhaps He won't, but there is much information on
AM reception out there in the old publications which mainly lay slowly rotting

in university basement achives if they havn't all been chucked out years ago.
I spent quite some time reading all I could and my copied paper files consist
of a couple of hundred sheets.

Why the heck would I ever want to re-invent the wheel with AM?

Better to consider the wisdom of the past before deciding on something novel.

The tubed tuners are in the minority in my files.

The commercialisation of the complex synchronous receiver types
was exceedingly limited, because when such receivers were concieved,
nearly everyone was farewelling AM for serious listening,
and going to FM.
But AM was good for the cricket, football, news, talkback and pop trash.
Rarely if ever was there any Bethoven.
And nearly everyone started using cheap japanese SS portables
in 1965. There was 3kHz of BW, if you were lucky.

I think one can get ceramic filters with 20 kHz of BW, -3 dB,
Murata part number CFU455E2 offers -6 dB at +/- 12.5 kHz.
The attenuation 10 kHz away from the -3 dB point is 90 dB.
These are usually low impedance input devices, maybe 1 kohm, so they need to
be driven with an untuned IF transformer with a low impedance secondary, or
cathode follower.
Don't apply DC to any of the pins on ceramic filters.

Patrick Turner.

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

7. It has been suggested that using a 2 MHz IF frequency would allow
wider bandwidth than the standard 455 kHz IF frequency. I fail to
see why
this should be true.

Because for the same Q value, the pass band would be 4 times wider

Where is it written that the same loaded Q must be used for both filters?
If you can change the center frequency, why can't you change the loaded Q?

The lower the Q, the more IFTs required for a given amount of pass band and
attenuationout of band.

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are.

Sure. But the same Q would give wider BW at 2 MHz.
I have not ever done this, so I guess at what the final response could be.



There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.



To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

One One
455 kHz IFT 2.0 MHz IFT
Q = 15.167 Q = 66.667

Fc-60 kHz -24.30 dB -24.18 dB
Fc-50 kHz -21.22 dB -21.09 dB
Fc-40 kHz -17.56 dB -17.42 dB
Fc-30 kHz -13.22 dB -13.07 dB
Fc-20 kHz -8.72 dB -8.60 dB
Fc-15 kHz -7.09 dB -7.02 dB
Fc-10 kHz -6.27 dB -6.24 dB
Fc-05 kHz -6.04 dB -6.04 dB
Fc kHz -6.02 dB -6.02 dB
Fc+05 kHz -6.03 dB -6.03 dB
Fc+10 kHz -6.19 dB -6.22 dB
Fc+15 kHz -6.86 dB -6.96 dB
Fc+20 kHz -8.34 dB -8.50 dB
Fc+30 kHz -12.75 dB -12.95 dB
Fc+40 kHz -17.15 dB -17.32 dB
Fc+50 kHz -20.88 dB -21.01 dB
Fc+60 kHz -24.01 dB -24.12 dB

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

The equality in performance depends on a large Q difference, with
544 kHz Q much lower than 2MHz Q to get the same BW.

The Q of a typical 455 kHz IFT is higher than you have indicated, because
the impedance of the LC circuit at Fo is required to be high to suit
pentode loading, and to get high gain.

If the Q was real low, and hence the Fo impedance, you
would probably need 3 IFTs.

I have never tried 3 very damped IFTs.




Within reason, for bandwidths typical of audio
receivers, you should be able to build a filter at 455 kHz that has
effectively the same response as a 2 MHz filter. There is no need to
throw out the 455 kHz IF just to get wide bandwidth.

Its difficult to make a 455kHz typical old IFT produce a nice flat topped
20 kHz wide BW. Its either pointy nosed, undecoupled, or flat
topped, critical
coupled,
or over critical or rabbit eared.
I have tried all that.

So you have tried all that and rejected the "pointy nosed", "flat topped",
and "rabbit eared" response curves. I am left to wonder what sort of
response curve you were looking for? Why not settle for a nice "flat
topped" response curve and be done with it?

I didn't say I had rejected the flat topped critical coupled IF response.

Then what did you say? You said you had "tried all that" but now it
appears that you were telling a little fib and hadn't actually tried a 455
kHz IF designed to produce the desired response.

What I said was what I said.
You are confused.

Build a radio with 2MHz and measure it, maybe it works better.

Just don't knock the idea before trying it, or condemn the idea
with postulations about what might be.
These things must be tried and measured, to really know.

Patrick Turner.



Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/